Method of measuring the concentration and/or size of suspended particles by forward scattering of light

ABSTRACT

A method for measuring the concentration and/or size of illuminated particles suspended in a gas by means of the scattered light process. The suspended particles are illuminated by passing them through a focused laser beam in the vicinity of its focal plane. The light that is scattered from the particles in the direction of propagation of the laser beam within an angle of + OR - 15* with respect to the focal line of the laser beam are then evaluated by a photodetector device to determine the desired parameter. The intensity of the laser beam is reduced if the anode current of the photodetector exceeds a preset limit. The output signal of the photodetector device is fed to a potentiometer which has a logarithmic resistance per unit of its length. The output of the potentiometer is fed to a fixed single channel pulse height analyzer, which records all the pulses above a predetermined threshold value and transmits them to a counter.

United States Patent 301 et al.

[54] METHOD OF MEASURING THE CONCENTRATION AND/OR SIZE OF SUSPENDEDPARTICLES BY FORWARD SCATTERING OF LIGHT [72] Inventors: Jan 801,Kelkheim; Werner lleinu; Josef Gebhardt, both of Frankfurt; WernerLetachert, Oberstedten, all of Germany [73] Assignee: Bundesmlnister furWissenshaftllche Forschung, Bonn, Germany [22] Filed: Oct. 10, 1969 [21]App1.No.: 865,404

[30] Foreign Application Priority Data Oct. 10, 1968 Germany ..P 18 02269.0

[52] US. Cl. ..250/2l8, 356/102, 356/103 [51] Int. Cl. ..G0ln 21/26 [58]Field otSearch ..356/102, 103, 210, 207, 208; 250/218, 222

[56] References Cited UNITED STATES PATENTS 3,389,259 6/1968 Zarem..250/20l 3,418,479 12/1968 Schmitt ..356/223 X 3,451,755 6/1969Silverrnan .....356/103 X 3,515,482 6/1970 Garrow ..356/ 103 3,528,7429/1970 Dobbs ..356/ 103 OTHER PUBLICATIONS German Printed Application:1,1 15,477 Stetter et al. 8- 4- 60.

1 1 Feb. 29, 1972 DeSilva. A. W.; et al. Observation of Thompson andCooperative Scattering of Ruby Laser Light by a Plama," Nature, Vol.203, No. 4952 pp. 1321- 22, 8- 26- 64.

George, T. V., et al. Scattering of Ruby Laser Beam by Glasses,"Physical Review Letters, Vol. 1 1, No. 9, I 1- 63, pp. 403- 5.

Kratohvil, J. P., Light Scattering," Analytical Chemistry. Vol.36, No.5, April 1964. PP- 458R- 72R.

Woodward, P. H., He- Ne Laser as Source for Light ScatteringMeasurements," Applied Optics, Vol. 2, No. 11, Nov. 1963. Pp. 1205- 07.

Primary Examiner-James W. Lawrence Assistant Examiner-D. C. NelmsAttorney-Spencer & Kaye [57] ABSTRACT A method for measuring theconcentration and/or size of illuminated particles suspended in a gas bymeans of the scattered light process. The suspended particles areilluminated by passing them through a focused laser beam in the vicinityof its focal plane. The light that is scattered from the particles inthe direction of propagation of the laser beam within an angle of 115with respect to the focal line of the laser beam are then evaluated by aphotodetector device to determine the desired parameter. The intensityof the laser beam is reduced if the anode current of the photodetectorexceeds a preset limit. The output signal of the photodetector device isfed to a potentiometer which has a logarithmic resistance per unit ofits length. The output of the potentiometer is fed to a fixed singlechannel pulse height analyzer, which records all the pulses above apredetermined threshold value and transmits them to a counter.

7 Claims, 2 Drawing Figures SE M HOTODE TECT OR 4 rm 1 E, f

DEFLECTING mason METHOD OF MEASURING THE CONCENTRATION AND/ OR SIZE OFSUSPENDED PARTICLES BY FORWARD SCATTERING OF LIGHT BACKGROUND OF THEINVENTION The invention relates to a method for measuring theconcentration and/or size of illuminated suspended particles by thescattered light technique.

For measuring the concentration and/or size of suspended particles,equipment has been on the market for a long time which operates by thescattered light technique and supplies measured results very quickly. Inthese devices, the intensity of the light scattered by the particlesacts as a measure of the particle size and the number of counts per unittime is a measure of the concentration. However, closer examinationshows that these results contain errors, because the readings dependupon the optical qualities of the particles to be investigated. This isa disadvantage especially when aerosols with particles which differ withrespect to their optical qualities are to be investigated; i.e.,so-called mixed aerosols.

The calibration curves supplied along with this equipment, moreover,refer only to spherical particles, whereas investigations of technicaldust must be performed mostly on completely irregular particles. Thisform factor is not taken into account at all by the familiar equipment.

SUMMARY OF THE INVENTION The purpose of the present invention is tocreate a method of the type mentioned initially which enables themeasurement of the concentrations and/or size of suspended particles ina comparatively large range of particle sizes and concentrations. Thesuspended particles in this case are to have diameters between 0.1 and10 pm; their concentration in the carrier medium is to be between andparticles per cm Moreover, the measured results are to be practicallyindependent of the optical qualities (refraction index, absorptivity)and the form of the particles.

In the present invention, the problem has been solved in such a way thatthe suspended particles are illuminated by a laser beam and that theforward lobe of the scattering light is evaluated within an angle of Thelaser beam is focused and the suspended particles are caused to passthrough the focused laser beam in the area of the focal line.

The use of a laser beam in the present invention establishes theprecondition for evaluation of only the light scattered within theforward lobe. The intensity of the light scattered in the small anglearea of the forward lobe is influenced least by the optical qualities ofthe suspended particles. In addition, the generally irregular form ofthe suspended particles within the angular area of the forward lobe hasthe smallest influence on the measure result. In this way, an exactderermination of the quantities to be measured is safeguarded.

The increase in accuracy and range of measurement of the methodaccording to the invention achieved through the use of a laser beam isdue to the following qualities of laser light:

The intensity of illumination is comparatively high. This increases thesensitivity and allows small particles to be detected. The pointcharacteristics of the focused laser as a light source permit the exactillumination of a relatively very small measured volume, i.e., it ispossible to measure high aerosol concentrations. The laser beam has asmall angle of aperture. In this way, a small illumination aperture inthe measurement volume can be achieved which, in turn, provides an exactdefinition of the forward lobe, or area of narrow forward scattering,and pennits a clean separation of the unscattered from the scatteredlight.

In order to be able to process in a conventional secondary electronmultiplier the intensity of the scattered light, which covers eightpowers of 10, in the envisaged range of particle sizes between 0.l and10 um and above, it is provided for in the present invention to reducethe intensity of the laser beam when the anode current of the photodetector exceeds a preset limit.

In the present invention, this reduction of intensity of the laser beamis achieved by triggering a Kerr or Pockel cell set up in the laser beamby the anode of the photo detector.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows schematically the beampath of the beam in an optical particle spectrometer for execution ofthe method according to the present invention.

FIG. 2 shows a block diagram of the electrical evaluation systemarranged after the secondary electron multiplier of FIG. I and of thetriggering of a Kerr or Pockel cell set up in the laser beam.

DESCRIPTION OF THE PREFERRED EMBODIMENT A laser beam 2 emerging from alaser 1 acting as a light source is propagated from left to right inFIG. I and focused onto a disk diaphragm B, by means of a lens L,. Thedisk diaphragm B, in turn is enlarged by a lens L and imagedastigmatically. In a measurement volume V, the laser beam 2 is focusedto a focal point defining a focal plane line. The distance between themeasurement volume V and the lens L should be as large as possible.Additional diaphragms B, are set up between the disk diaphragm B, andthe measurement volume V along the path of propagation of beam 2. Inthis way, the measurement volume V is shielded from the deflected lightat the disk diaphragm B, and the scattered light of the lens L Particlescarried along in a test air stream 0 pass through the laser beam 2 inthe measurement volume V, giving off a pulse of scattered light. Of thisscattered light, the fraction occuring in the narrow forward range isrecollected by lenses L, and L.,, the respective scattering particlebeing imaged in a disk diaphragm B The scattered light penetratingthrough the disk diaphragm B reaches a photodetector SEM. It supplies anelectric signal which is proportional to the intensity of the scatteredlight. The particle size thus results from the relationship between theintensity of scattered light detected and the particle size d not shown.

After the measured volume V, the nonscattered direct laser beam 2 passesthrough clearances in a lens L and a deflecting mirror S to reach alight trap 3 and in, this way, is separated from the path of the beamsimaging the particles. Usually, the signal at the photo detector isdirectly fed into a multichannel pulse analyzer with a linearsubdivision of channels. This results in a greatly distorted scale forthe particle size d, which is adapted to the resolution of the particlesize spectrometer and not over the entire recorded range of particlesizes.

This disadvantage is avoided by simple means in the method according tothe present invention. According to FIG. 2, the signal voltage of thephoto detector SEM is divided after amplification in an amplifier V, andpaxed through a potentiometer P having logarithmic resistance per unitlength. The voltage at the tap of the potentiometer P is fed through anarmplifier V to a fixed single channel pulse height analyzer D,. Thisrecords all the pulses with a voltage Ei-AE and transmits them to acounter 2,. At each position of the tap of the potentiometer P, aspecific particle size d is registered. On the basis of the familiarrelationship between the intensity of the light of an illuminatedparticle scattered into the narrow forward lobe and the particle size d,the tap of the potentiometer P in the different ranges of size can thenbe calibrated in term of particle sizes with the respective logarithmicscale.

All pulses occuring at the output of the amplifier V, are separated fromthe noise background by a threshold switch S and fed to a counter 2;. Inthis way, the concentration of the aerosol to be analyzed is determinedand controlled.

Since the intensity of the scattered light changes with the fourth tosixth power of the particle diameter d, the photo detector would beoverloaded by the passage of the largest occurring particles through themeasurement volume. To avoid this, the intensity of the laser beam 2,which has been polarized from the outset, is reduced by a Pockel or Kerrcell K controlled by the output of the photo detector when the photocurrent exceeds an adjustable setting. For this purpose, the signal iscollected at the output of the photodetector SEM and compared with apreselectable voltage E, by means of a discriminator D If the voltagederived from the photo detector exceeds this voltage E,,, a Pockel orKerr cell is triggered by an amplifier V in such a way that theintensity of the laser beam 2 is reduced in the weli-known manner. Forthis purpose, the Kerr cell K is set up in the laser beam 2.

The rise of a signal at the photodetector occurs over a period of a fewmicroseconds; by contrast, a Pockel or Kerr cell is able to switchwithin a period shorter than 0.1 microsecond so that the desiredprotection from overloading of the photodetector and, especially of thephoto cathode is safeguarded. This overload protection is activated onlyfor particles above some pm.

We claim:

1. A method for measuring both separately and simultaneously theconcentration and size of suspended particles, comprising the steps of:

a. illuminating the particles by passing them through a focused laserbeam at a point substantially in a focal plane defined by the laserbeam; and

b. detecting substantially all of the light scattered from the laserbeam by the particles and substantially none of the unscattered lightwithin an angle of $15 with respect to the direction of propagation ofthe laser beam.

2. The method of claim 1, wherein said detecting step includes the stepof detecting the scattered light with a photodetector and reducing theintensity of the laser beam if the anode current of the photodetectorexceeds a predetermined value.

3. The method of claim 2, further including the step of triggering aKerr cell to reduce the intensity of the laser beam.

4. The method of claim 2, further including the step of triggering aPockel cell to reduce the intensity of the laser beam.

5. The method of claim 2, wherein the detecting step further includesthe step of feeding the output of the photodetector to a potentiometerhaving logarithmic resistance per unit of its length.

6. The method of claim 5, wherein the detecting step further includesthe step of feeding the output of the potentiometer to a fixed singlechannel pulse height analyzer.

7. The method of claim 6, wherein the potentiometer has a tap, and iscalibrated in particle sizes with a logarithmically subdivided scale,and the feeding from the potentiometer is at the tap.

UNITED STATES PATENT OFFICE QERTH ICATE U51 CORRECTWN Pa NO. 3 I 646Dated February 29th, 1972 In )Jan Bol, Werner Heinze ,JosefGebhardt.werner Letschert It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the heading of the patent, line 8, change "Wissenshaftliche" towissenschaftliche. Column 1, line 6, after "The" insert present.

Signed and sealed this 3rd day of October 1972.

(SEAL) Attest:

EDWARD MQFLETCHERJR, ROBERT GOTTSCHALK Attesting Officer Commissioner ofPatents ORM PO-105O (10-69) USCOMM-DC 50376-5 69 U 54 GOVERNMENTPRINTING OFFICE: 1965 O365-334

1. A method for measuring both separately and simultaneously theconcentration and size of suspended particles, comprising the steps of:A. illuminating the particles by passing them through a focused laserbeam at a point substantially in a focal plane defined by the laserbeam; and b. detecting substantially all of the light scattered from thelaser beam by the particles and substantially none of the unscatteredlight within an angle of + OR - 15* with respect to the direction ofpropagation of the laser beam.
 2. The method of claim 1, wherein saiddetecting step includes the step of detecting the scattered light with aphotodetector and reducing the intensity of the laser beam if the anodecurrent of the photodetector exceeds a predetermined value.
 3. Themethod of claim 2, further including the step of triggering a Kerr cellto reduce the intensity of the laser beam.
 4. The method of claim 2,further including the step of triggering a Pockel cell to reduce theintensity of the laser beam.
 5. The method of claim 2, wherein thedetecting step further includes the step of feeding the output of thephotodetector to a potentiometer having logarithmic resistance per unitof its length.
 6. The method of claim 5, wherein the detecting stepfurther includes the step of feeding the output of the potentiometer toa fixed single channel pulse height analyzer.
 7. The method of claim 6,wherein the potentiometer has a tap, and is calibrated in particle sizeswith a logarithmically subdivided scale, and the feeding from thepotentiometer is at the tap.